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1.1 Heart's electrical activity
The sinoatrial node (SA) is a concentration of specialized cells with electrical activity found in
the wall of the right atrium which send an electrical signal that initiates atrial depolarization and
the contraction of the cardiac cavities known as the heartbeat, so it is regarded as a
natural/biological pacemaker.
The SA signal is detected by the atrioventricular node (AV) after a delay of approximately one-
tenth of a second and in turn produces an electrical stimulus that is transmitted through the
bundle in its right and left branches to the Purkinje fibers that are responsible for distributing the
electrical signal for all the muscle fibers of the ventricles. This signal causes ventricular
depolarization and thus contraction of the ventricles. Next, a period of about 2/3 of the total time
of the cardiac cycle occurs in which the fibers of all cavities are repolarized in preparation for the
next cardiac beat [2]. The sensors used to measure electrical potentials generated are known as
electrodes and are placed on the chest area around the heart. The measurement of voltage
between two of these electrodes is called derivation and records the electrical activity of the heart
from a certain angle. This activity is recorded in an electrocardiogram (EKG), generating the
electrical waveform that has its parts corresponding to P-wave atrial depolarization, the QRS-wave
which corresponds to ventricular depolarization, and T-wave which corresponds to the
polarization. The wave generated has low-frequency components caused by variations in heart
rate produced by pathologies such as tachycardia and bradycardia, medium frequency
components corresponding to the polarization of the membrane of heart muscle cells, and high
frequency components from the depolarizations that trigger myocardial contraction [3].
1.2 Mechanical functioning of the heart
From the mechanical point of view, the heart can be considered as two synchronized constant-
volume piston pumps, cycling 80,000-100,000 times each day (~8,000 liters displaced). The atria
produce a contraction which only marginally contributes to blood pressure and do not generate
the closure of any heart valves. The ventricles generate the principal components of blood
pressure, producing a pressure which in normal ranges varies between 0 and 110 mm Hg along
with a volume variation from 150 ml to 50 ml between relaxation and full contraction. The
contraction of the ventricles causes the closure of the mitral and tricuspid valves and the
relaxation of the same results in closure of the pulmonary and aortic valves. Heart valves are
diaphragm-type check valves roughly 24 mm in diameter for the mitral and tricuspid valves and 18
mm in diameter for the aortic and pulmonary valves.
A pumping cycle starts with a right ventricle almost full to its maximum relaxed size at the end
of previous cycle. Atrial contraction then occurs, giving a slight impulse to the blood entering the
ventricle through the mitral valve. This is followed by ventricular contraction with an increase in
pressure which causes the mitral valve to close and opens the pulmonary valve to the lungs. The
other atrial contraction and subsequent ventricular contraction produces the closure of the